Winding is a vital step in the process of making and converting web material. If the web is not wound properly, defects such as scratches can result which make the web unsuitable for finished product, particularly with photographic film and paper. A web must be wound properly so that it can be stored without deterioration; if not, the web may be unsuitable for saleable product. The total loss of a wound roll can be very costly.
The entrainment of air while winding can cause defects in a wound roll. Air entrainment occurs when a layer of air moving with the web becomes wound into the roll. Air entrainment can occur at any web speed but is a particular concern when winding at high speeds, for example, 100 feet per minute or greater. Generally, air entrainment increases as roll radius, air viscosity, and web velocity increase, while air entrainment generally decreases as web tension increases. Other factors affecting the amount of entrained air are web temperature, surface roughness, planarity, and thickness variation.
If the amount of entrained air is substantial, a lap of wound web may not make contact with a previous lap, and the newly wound lap may shift or move relative to the previous lap in a direction that is parallel to the axis of the core (generally known as surface skidding), especially as the diameter of the wound roll increases. Entrained air makes the roll unstable, causing the roll to shift or move dramatically relative to the core. For coated webs, such as emulsion coated photographic webs, shifting can cause scratching of the coating, resulting in unsalable or defective product. Additionally, shifting may cause the web edges to contact the winding machinery, possibly resulting in severe damage to the roll such that it would be unsuitable for subsequent processes.
Air entrainment may also result in another winding problem called cinching. Cinching is the relative motion of one lap to another in the circumferential direction. Cinching occurs when in-roll pressure and, therefore, the frictional force, is reduced due to the air entrainment. The torque used in winding the roll eventually overcomes the reduced frictional force between the two laps and relative motion occurs. Scratching of the web and/or film emulsions can result, as can axial shifting.
During storage, entrained air within a roll will eventually leak out of the roll. If the roll is stored with the core axis in a vertical orientation, the roll can loosen and shift downward due to gravity. The lower web edges can buckle, causing a defect referred to as edge cockle. If the roll is stored with the core axis oriented horizontally, air will still leak out and the roll may sag in the center due to gravity, resulting in permanent stretching deformation. In both cases, the roll may not be suitable for unwinding in subsequent processes.
One method for reducing air entrainment is to carry out the entire winding process in a vacuum chamber. However, such a solution would be extremely costly and is not realistic.
Another method for reducing air entrainment is to increase the winding tension. However, winding machines are generally limited by drives to a maximum given amount of tension, which may not be sufficient to reduce air entrainment at high speeds for quality winding.
Another method for reducing the amount of entrained air is to use a contact roller or pressure roller in contact with the roll being wound, to squeeze out air as the roll is being wound. With such a method, the pressure roller must accommodate features of the web material, such as variations in thickness, while still providing contact with the center of the roll to reduce air entrainment. In addition, it is preferred to have a pressure roller with a length substantially equal to the width of the web material. As illustrated in FIG. 1, a contact roller or pressure roller 10 is in contact with a roll 11 as it is being wound. A nip, shown generally as nip 12, is formed between pressure roller 10 and winding roll 11, and sufficient force is applied to squeeze out the air at nip 12 when web material 13 is wound onto core 14. The use of pressure roller 10 increases the wound-in tension of the roll, allowing a reduced amount of air to be entrained during the winding process. Generally, pressure rollers 10 have a hard metal surface, such as stainless steel or aluminum, or have a single resilient covering with the hardness of approximately 50 to 70 durometer Shore A.
Pressure roller 10 may be applied using various configurations. For example, in pressure roller assisted center winding, a force F is applied to the pressure roller and the core is driven about a fixed center while the pressure roller is idling on a pivot or slide; the pressure roller rotates at the speed of the moving web due to frictional force applied to the pressure roller by the moving web. Similarly, in surface winding, the core can idle on a fixed center with the pressure roller being driven about pivot or slide. In a further configuration referred to as reel winding, the core idles on a non-fixed center position with the pressure roller driven about a fixed center, and the force F is applied to the core. In the dual driven winding configuration, the core and pressure roller are both driven at a substantially equal surface speed, or the core is driven while the pressure roller is driven at a surface speed slower than the core. With double drum winding, the roll is cradled between two driven pressure rollers, and the core is idling.
The widthwise thickness variation of web 13 can affect the winding of a roll. Typically, there is some variation in widthwise thickness, particularly with web materials such as cellulose triacetate, polyethylene terephthalate, or polyethylene naphthalate. As illustrated in FIG. 2, if such a thickness variation persists in the lengthwise direction of the web, known as a gage band 16, then, when the web is wound onto a core, gage band 16 lies upon itself as subsequent laps of web are wound. This produces a hard thick band or streak of extremely high localized pressure, typically known as a hardstreak. A hardstreak may cause detrimental effects to the web such as abrasions, deformations, and chemical and/or physical changes to the web material.
Referring to FIG. 3, U.S. Pat. No. 4,934,622, commonly assigned, and incorporated herein by reference, discloses a method of overcoming the problems of gage bands by knurling 18 the margins of the web material, so that the protuberances produced by the knurling are higher than any gage band likely to be encountered in normal manufacturing. When the knurled web is wound onto a core, the knurled margins overlap, and, it is in this area that the high pressure between adjacent turns is encountered. The buildup of thickness due to the knurled margins is commonly referred to as a knurl tire 20, as illustrated in FIG. 3. The knurl tire 20 of an uncoated web material can typically be as high as 0.125 inches (3.175 mm) above the body of the roll, while knurl tires of a coated web material can typically be as high as 0.0625 inches (1.5875 mm) above the body of the roll. As the roll diameter increases, these heights may increase.
FIG. 4 illustrates a coated web material (that is, web material with one or more layers or coating 22) having knurls 18 and coated margins 24 which are thicker than the remainder of the coated web material. This is referred to as an edgebead 26, and may be magnified when more than one coating is provided or if the knurled margins are coated. An illustration of edgebeading is shown in FIG. 4 where the margins 24 of the coating 22 are thicker than the center portion of the coating. When such thicker coated margins are wound onto a core, hardstreaks may occur, forming an edgebead tire. This is similar to a thin knurl tire, however, the width of a coating edgebead is much less than the width of a knurl. If the thickness of the portion of web material which is coated is of a similar thickness as the knurled edge, the web material will wind on the edgebead, and high localized pressures may result. If the edgebead tire is narrow, it will axially buckle under sufficient pressure, possibly causing the roll to cinch or shift or a combination of the two.
Accordingly, it is preferred that a roll be wound with reduced air entrainment, yet a pressure roller should accommodate existing hardstreaks, knurl tires, and edgebead tires. That is, pressure rollers need to contact hardstreaks, knurl tires and edgebead tires while still providing contact with the center of the roll to reduce air entrainment. In addition, it is preferred to have a pressure roller with a length substantially equal to the width of the web material.
U.S. Pat. No. 5,039,023 discloses a contact roll and two air displacement rollers to squeeze out the air boundary layers during winding. The contact roll and one of the air displacement rollers have a hard smooth surface layer with an average peak-to-valley height Ra of less that 0.4 microns and a Brinell hardness greater than 10 HB 2.5/62.5. Such rollers are expensive to produce and will not contact the entire roll surface in the presence of knurl tires, edgebead tires, or hardstreaks. In addition, contacting hardstreaks with a hard surface, particularly at high speeds, may create even higher localized pressures, resulting in damage to the web.
U.S. Pat. No. 3,622,059 discloses a transport roller including a resilient roller filled with glass spheres that maintain a light and uniform pressure across the nip. Such a roller is believed to be unsuitable for high speed applications, for example, for speeds greater than 1000 feet per minute.
Accordingly, there exists a need for a pressure roller which reduces air entrainment, and accommodates web material having hardstreaks, knurl tires, and edgebead tires. The present invention solves these problems by providing a pressure roller with variable durometer.